Children are the most naturally physically active human beings; reduced physical activity is a cardinal sign of childhood disease, and exercise testing provides mechanistic insights into health and disease that are often hidden when the child is at rest. Despite this, and because data analytics and testing protocols have failed to keep pace with enabling technologies and computing capacity, biomarkers of fitness and physical activity have yet to be widely incorporated into translational research and clinical practice in child health. The goal of this project is to address the obstacles that have impeded optimal use of cardiopulmonary exercise testing (CPET) in children. Aim 1 is to rethink and transform current clinical research applications of typical CPET in children by novel implementation of breath-by-breath technologies and data analytic approaches. Aim 2 is to develop new exercise testing protocols (multiple brief exercise bouts) that more closely mimic real-life patterns of physical activity and, in so doing, better assess relevant pathophysiology. Aim 3 is to identify and overcome logistical issues that have limited multicenter studies involving exercise biomarkers. Our use-cases, sickle cell disease and cystic fibrosis, highlight how very different diseases can impair exercise and physical activity. The value of the new CPET approaches will be analyzed in a variety of ways, including by: 1) established indexes of health (e.g., body composition), 2) habitual physical activity, an emerging metric of overall health, and 3) novel exercise-responsive gene expression signals in the circulating blood. In combination with healthy children as comparisons, our project will delineate the use of CPET biomarkers across a broad spectrum of pediatric health and disease. We will take advantage of the grant cycle and study cohorts prospectively as our participants grow and mature from Tanner 2?3 to 5, roughly a 3-year interval. This provides the unique opportunity to study CPET longitudinally in health and disease over a critical period of growth. Our team represents the diverse and challenging array of academic health centers and affiliated stand-alone children?s hospitals often involved in multicenter pediatric translational research: 1) University of California at Irvine Institute for Clinical and Translational Science, 2) Northwestern University Clinical and Translational Sciences Institute, and 3) the Southern California Clinical and Translational Science Institute, Children?s Hospital of Los Angeles, University of Southern California. Each of these centers is committed to underserved populations. In addition, we will collaborate with the North American Society of Pediatric Exercise Medicine and the American College of Sports Medicine, each dedicated to advancing exercise-as-medicine in children. Through our proposed: 1) innovations in data analytics of commonly used CPET, 2) developing new testing paradigms, 3) providing the research community with essential data to determine statistical power and analytic approaches specific for exercise biomarkers, and 4) improving interoperability in multicenter trials, the project will stimulate child health clinical research in which exercise biomarkers are used as key outcome variables.